This invention relates to selecting the modulation scheme and/or the amplification scheme that is to be used in a data transmitter, for example a radio transmitter.
FIG. 1 is a schematic view of an example of a radio transmitter. The transmitter has an input 1 at which it receives a stream of data to be transmitted. That data is modulated by a modulation stage 2. The output of the modulation stage is amplified by an amplifier stage 3 and the resulting signal passes to an antenna 4 from which it is transmitted.
The modulation stage 2 is capable of modulating signals according to a number of modulation schemes. The scheme that is to be used is selected by a control unit 5. These schemes may differ in their data rate, their power consumption or their sensitivity to interference. The control unit is programmed to select a scheme in response to conditions in the system: for example, the current error rate between the transmitter and a receiver.
The amplifier stage 3 is capable of applying a range of levels of amplification. The level of amplification that is to be used is selected by the control unit 5, in response to conditions such as the estimated loss over a link between the transmitter and a receiver, or in response to power control signals received from the receiver. The variation in amplification may be achieved by a single amplifier unit or by a combination of a pre-amplifier 6, which is always in-circuit, and a power amplifier 7 which can be switched in when high gain is required. In either case, the response of the amplifier is typically less accurate at higher levels of gain.
This inaccuracy may have different effects on different modulation schemes. Some schemes may be relatively unaffected whereas other schemes may become distorted to the extent that at high levels of gain they cannot be decoded at the receiver. The non-linearity of the amplifier may cause spectral distortion such that under some modulation schemes the transmitter transmits significant levels of power outside its permitted frequency band. In a typical application, two devices may be interoperating at short range using a high data rate modulation scheme and without using a power amplifier. As the range increases, the two devices may communicate using a standard power control mechanism (or by other means) and agree that the transmitted power level in the link should be raised. This may involve taking a power amplifier into use, which may cause distortion with a high data rate modulation scheme.
One way to resolve this is to use an amplifier stage that has a better linearity. However, this may increase cost and occupy a greater area on the circuit board or integrated circuit on which it is implemented.
There is therefore a need for an improved method of controlling a transmitter.